Production of ceramic materials having improved flexural strength has been the subject of research by many investigators, because poor flexural strength is a most serious disadvantage of ceramic materials, and if ceramic materials having improved flexural strength can be developed, they can be effectively used in the fabrication of cutting tools, as synthetic bone materials, as parts for internal combustion engines, and so forth.
For example, Japanese Patent Application (OPI) No. 140762/80 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") discloses "zirconia-base cutting tool materials" comprising ZrO.sub.2 partially stabilized with oxides of Y, Ca, Mg, etc., in which the total fraction of tetragonal and cubic ZrO.sub.2 is from 60 to 95% by weight. J. S. Reed et al., Ceramic Bulletin, Vol. 55, page 717 (1976) describes that high strength ZrO.sub.2 sintered bodies can be obtained by sintering fine powdered ZrO.sub.2 which is prepared by co-precipitating a mixture of ZrOCl.sub.2 and YCl.sub.3, calcining the thus-formed powder, and stabilizing with Y.sub.2 O.sub.3.
These ceramic materials, however, are not completely satisfactory in strength, and it has, therefore, been desired to further increase the strength, because it is expected that such improved ceramic materials would have a wider variety of uses.